A very cute use for egrep.

Local UUG is currently in the midst of a cascade on funky shell tricks; I found this one from Russell Stuart, particularly cool.

tr "[:print:]" "[x*]" < FILE | egrep -vno '^(..+)\1+$'

[H-CHAT] What's your top three shell tricks?

The thread explains what it does, and I would probably replace the .'s in the egrep pattern with x's; still very cute.

It can also provide a good example of when cs theory can occasionally be useful: Given the provided solution uses a gnu-extensions to regex(7), can this be done with a pure POSIX regular expression?